Polymer crystallization nucleation

Low-shrink polypropylene fibers requires presence of additive that nucleates polymer crystals within the target polypropylene after exposure to sufficient heat to melt the initial pelletized polymer and allowing such a melt to cool." The preferred rigidifying compounds include dibenzylidene sorbitol based compounds, as well as less preferred compounds, such as sodium benzoate, certain sodium and lithium phosphate salts (such as sodium 2,2 -methylene-bis-(4,6-di-tert-butylphenyl)phosphate, otherwise known as NA-11)."... 

Hu W, Cai T (2008) Regime transitions of polymer crystal growth rates molecular simulations and interpretation beyond Lauritzen-Hoffman model. Macromolecules 41(6) 2049-2061 Hu W, Frenkel D (2004) Effect of metastable liquid-liquid demixing on the morphology of nucleated polymer crystals. Macromolecules 37(12) 4336-4338 Hu W, Frenkel D (2005) Polymer crystallization driven by anisotropic interactions. Adv Polym Sci 191 1-35... 

Hu W, Frenkel D. Effect of metastable liquid-liquid demixing on the morphology of nucleated polymer crystals. Macromolecules 2004 37 4336 338. 

Insoluble additives may nucleate polymer crystallization, while high concentrations of insoluble additives may restrict crystallization. Soluble additives at low concentrations should not affect the crystallinity of the polymer. High concentration levels of an additive may change the properties of the polymer by accident or by design. 

The greater the undercooling, the more rapidly the polymer crystallizes. This is due to the increased probability of nucleation the more supercooled the liquid becomes. Although the data in Fig. 4.8 are not extensive enough to show it, this trend does not continue without limit. As the crystallization temperature is lowered still further, the rate passes through a maximum and then drops off as Tg is approached. This eventual decrease in rate is due to decreasing chain mobility which offsets the nucleation effect. 

Since interactions at the molecular level between polymer components in the blends occur only in the amorphous phase, it is reasonable to assume that these effects are due to kinetic factors and, in particular, to the influence of a polymer component on the nucleation or crystallization kinetics of the other one. 

The importance of twinned crystals in demonstrating that nucleation is the relevant growth mechanism has been realized since 1949 [64, 99]6. They were first investigated extensively in polymer crystals by Blundell and Keller [82] and they have recently received increased attention as a means of establishing, or otherwise, the nucleation postulate for lamellar growth [90, 91, 95,100-102]. The diversity of opinion in the literature shows that it is very difficult to draw definite conclusions from the experimental evidence, and the calculations are often founded upon implicit assumptions which may or may not be justified. We therefore restrict our discussion to an introduction to the problem, the complicating features which make any a priori assumptions difficult, and the remaining information which may be fairly confidently deduced. 

Despite the above comments on the application of roughening to thin polymer crystals it is certainly possible that thermal effects could cause a significant number of surface steps, and that this is all that is required to lead to a breakdown of the nucleation argument. 

Heterogeneous nucleation of polymer crystallization resembling a visualized metaphor compare the way meat is prepared in an oriental way shish - kebab. 

Polymers crystallize from the molten state by the two-step process of nucleation and crystal growth. Nucleation initiates crystallization, followed by the addition of linear chain segments to the crystal nucleus. 

Temperature has a complex effect on crystallization rate. Initially, as the temperature falls below the equilibrium melting temperature, the crystallization rate increases because nucleation is favored. However, as the temperature continues to fall, the polymer s viscosity increases, which hampers crystallization. As a rule of thumb, a polymer crystallizes fastest at a temperature approximately mid-way between its glass transition temperature and its equilibrium melting temperature. 

Polymer crystallization is usually initiated by nucleation. The rate of primary nucleation depends exponentially on the free-energy barrier for the formation of a critical crystal nucleus [ 110]. If we assume that a polymer crystallite is a cylinder with a thickness l and a radius R, then the free-energy cost associated with the formation of such a crystallite in the liquid phase can be expressed as... 

Interestingly, this barrier does not depend on chain length. This result coincides with experimental observations on the primary nucleation rate of bulk polymers [128-130]. For secondary nucleation of crystallization on a smooth growth front, a similar free-energy expression can be obtained for 2D nucleation ... 

Polymer crystallization is usually divided into two separate processes primary nucleation and crystal growth [1]. The primary nucleation typically occurs in three-dimensional (3D) homogeneous disordered phases such as the melt or solution. The elementary process involved is a molecular transformation from a random-coil to a compact chain-folded crystallite induced by the changes in ambient temperature, pH, etc. Many uncertainties (the presence of various contaminations) and experimental difficulties have long hindered quantitative investigation of the primary nucleation. However, there are many works in the literature on the early events of crystallization by var-... 

Price F (1969) Nucleation in polymer crystallization. In Zettlemoyer AC (ed) Nucle-ation. Marcel Dekker, Inc, New York... 

On the basis of the concept described above, we propose a model for the homogeneous crystallization mechanism of one component polymers, which is schematically shown in Fig. 31. When the crystallization temperature is in the coexistence region above the binodal temperature Tb, crystal nucleation occurs directly from the melt, which is the well-known mechanism of polymer crystal nucleation. However, the rate of crystallization from the coexistence region is considered to be extremely slow, resulting in single crystals in the melt matrix. Crystallization at a greater rate always involves phase separation the quench below Tb causes phase separations. The most popular case... 

Fig. 31 Structural formation model for the initial stage of polymer crystallization [19], N G nucleation and growth of oriented domains, SD spinodal decomposition into oriented and unoriented domains, Tb, Ts, and Tx bimodal, spinodal, and crystallization temperatures, respectively I isotropic, N smectic, and C crystalline...

The technique of self-nucleation can be very useful to study the nucleation and crystallization of block copolymers that are able to crystallize [29,97-103]. Previous works have shown that domain II or the exclusive self-nucleation domain disappears for systems where the crystallizable block [PE, PEO or poly(e-caprolactone), PCL] was strongly confined into small isolated MDs [29,97-101]. The need for a very large number of nuclei in order to nucleate crystals in every confined MD (e.g., of the order of 1016 nuclei cm 3 in the case of confined spheres) implies that the amount of material that needs to be left unmolten is so large that domain II disappears and annealing will always occur to a fraction of the polymer when self-nucleation is finally attained at lower Ts. This is a direct result of the extremely high number density of MDs that need to be self-nucleated when the crystallizable block is confined within small isolated MDs. Although this effect has been mainly studied in ABC triblock copolymers and will be discussed in Sect. 6.3, it has also been reported in PS-fc-PEO diblock copolymers [29,99]. 

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